1. Field of the Invention
This invention relates to material processing and particularly to methods of sintering ceramics, cermets, powdered metals and the like to enhance densification.
2. Prior Art
Certain covalent compounds, which have either low thermal expansion or high thermal conductivity, have been shown to exhibit desirable thermal shock characteristics. Typical of such compounds are silicon-based, nonoxide ceramics, such as Si.sub.3 N.sub.4 or SiC. Unfortunately, these materials have, heretofore, been extremely difficult to fabricate. Until recently, engineering components made of silicon-based, nonoxide ceramics were fabricated either by hot pressing or reaction sintering. However, reaction sintering forms a microstructure which precludes achievement of high strength and allows internal oxidation that eventually leads to cracking. Hot pressed silicon-based, nonoxide ceramics are strong and oxidation resistant, but hot pressing is an expensive fabrication technique and virtually eliminates the fabrication of complex shaped components.
Recently, silicon-based, nonoxide ceramics have been fabricated by cold compaction and sintering. These compounds generally are sintered with small amounts of Al.sub.2 O.sub.3, as well as other sintering aids, i.e. MgO, Y.sub.2 O.sub.3, etc. The Al.sub.2 O.sub.3 addition tends to improve the densification while reducing the weight loss during sintering by the formation of a low temperature glassy phase. However, the high temperature characteristics of the silicon-based, nonoxide ceramics are seriously degradated by the addition of such Al.sub.2 O.sub.3. Also, recently silicon-based nonoxide ceramics have been sintered under high pressure. However, this has been done only on a small scale.